Field of the Invention
The present invention relates to a gas sensor element for detecting a particular gas contained in gas to be measured, a gas sensor having the gas sensor element, and a method of manufacturing the gas sensor element.
Description of Related Art
A conventionally known gas sensor having a gas sensor element for detecting a particular gas contained in gas to be measured is, for example, an oxygen sensor installed in an exhaust flow path, such as an exhaust pipe, of an internal combustion engine and utilized in controlling combustion of the internal combustion engine through detection of an oxygen concentration in exhaust gas. The oxygen sensor has, for example, a tubular metallic shell and a plate-like gas sensor element held in the metallic shell.
The gas sensor element includes a longitudinally extending plate-like element body, and a protection layer formed of a porous material and provided on the surface of the element body. The element body includes a detecting section provided in a longitudinally forward region thereof and adapted to detect a particular gas contained in gas to be measured. The protection layer is provided in a longitudinally forward end region of the element body in such a manner as to cover at least the detecting section.
The protection layer is provided for protecting the element body. Direct adhesion of condensed water may break the heated element body by thermal shock. Thus, by means of provision of the protection layer for restraining direct adhesion of condensed water or the like to the element body, breakage of the element body can be restrained. The protection layer has a certain thickness for allowing condensed water to evaporate before reaching the element body.
A known method of forming the protection layer of the gas sensor element is a dipping process in which a forward end portion of the element body is dipped in slurry prepared by mixing ceramic powder, water, and a pore-forming agent (e.g., carbon powder) (refer to Patent Document 1). The slurry applied to the element body undergoes heat treatment and becomes the protection layer.